1. Field of the Invention
The present invention relates to a nozzle used in a continuous casting process for pouring a molten metal such as molten steel from a ladle into a tundish or from the tundish to a mold. More particularly, the present invention is concerned with a nozzle for use in continuous casting which is improved to prevent deposition of non-metallic inclusions in the molten metal to the surface of the nozzle wall defining the passage for the molten metal.
2. Description of the Related Art
In general, a nozzle for continuous casting has suffered from a problem in that particles of non-metallic inclusions such as alumina contained in the poured steel such as killed steel are undesirably deposited to the surface of the nozzle during continuous casting. As a consequence, the inside diameter of the nozzle port is progressively decreased to restrict and block the nozzle port, with the result that the pouring of the molten steel and, hence, the continuous casting are hampered.
In order to obviate this problem, it has been proposed to provide a porous portion in the surface of the nozzle port and to have an inert gas blown into the nozzle port through the porous portion, thereby preventing deposition of the non-metallic inclusions.
This method, however, is still unsatisfactory in that it is impossible to blow the inert gas over the entire area of the inner surface of the nozzle port. In addition, this method cannot be applied to some types of steel because voids tend to be generated in the cast slab.
Under these circumstance, attempts have been made to prevent deposition of non-metallic inclusions by improving the material of the surface region of the nozzle port, without relying upon the blowing of inert gas.
For instance, Japanese Patent Publication No. 2-12664 discloses a nozzle in which the inner peripheral region of the nozzle wall defining the nozzle port is made of a material containing 90 to 50 wt % of MgO and 10 to 50 wt % of C, while the outer peripheral region of the nozzle wall is formed from a sintered refractory material of Al.sub.2 O.sub.3 -C type.
On the other hand, Japanese Patent Laid-Open Publication No. 56-139260 discloses a nozzle in which the inner surface region of the nozzle wall, 3 to 15 mm thick as measured from the inner peripheral surface, is made, through a forming by a rubber press and sintering, from a material containing 5 to 80 wt % of boron and the balance one, two or more materials selected from the group consisting of graphite (C), alumina (Al.sub.2 O.sub.3), mullite (3A.sub.2 O.sub.3.2SiO.sub.2), zircon (ZrO.sub.2.SiO.sub.2), zirconia (ZrO.sub.2), metallic silicon (Si) and silicon carbide (SiC).
The nozzle disclosed in the aforementioned Japanese Patent Publication No. 2-12664 remarkably diminishes deposition of alumina-type inclusions and exhibits a large resistance against melt down by the heat of the molten steel, because the surface region of the nozzle port is made from a material containing 50 to 90 wt % of MgO. On the other hand, however, this nozzle tends to suffer from cracking due to a large thermal expansion coefficient as compared with conventional Al.sub.2 O.sub.3 -C nozzle. In addition, this nozzle exhibits inferior anti-spalling characteristics. Thus, the MgO-C type nozzle cannot be used with satisfaction because of a too small yield and because of large tendency of cracking particularly when the operating conditions are strict, although it provides an appreciable improvement in anti-erosion characteristics, as well as in prevention of blocking due to deposition of non-metallic inclusions.
The nozzle proposed in Japanese Patent Laid-Open Publication No. 56-139260 contains 5 to 80 wt % of BN and 20 to 95 wt % in total of one, two or more of additives such as C, Al.sub.2 O.sub.3, 3Al.sub.2 O.sub.3.2SiO.sub.2, ZrO.sub.2.SiO.sub.2, ZrO.sub.2, Si and SiC in the form of grains having a grain size not greater than 0.5 mm. When a coarse grains are used, part of the grain is melted down by the molten steel so as to progressively coarsen the inner surface of the nozzle wall defining the nozzle port so as to promote deposition of non-metallic inclusions. In consequence, the passage for the molten steel in the nozzle port is progressively restricted to impede safe continuous casting operation. Thus, the nozzle of this type can effectively prevent deposition of non-metallic inclusions only in the beginning period of casting, and cannot stand up against repeated cycles of continuous casting. Conversely, when the additive or additives used are in the form of fine grains, the strength of the inner surface region of the nozzle wall defining the nozzle port is small, so that the surface exhibits a large rate of melt down into the molten steel, as well as large rate of wear due to friction with the molten steel. Thus, the nozzle of this type undesirably allows mixing of the nozzle wall material into the molten steel and cannot withstand a long use and, therefore, cannot be used suitably for repeated cycles of continuous casting.